Obstructive Sleep Apnea (OSA), a syndrome that includes apnea, hypopnea and heavy snoring, causes sleep disruption that brings about serious health problems possibly including types of heart disease. OSA is caused by the collapse of portions of a person's airway passage. The treatment of choice for OSA is the administration of Continuous Positive Airway Pressure (CPAP) to keep the patient's airway open. The air, usually in the pressure range 4-20 cm H2O, is supplied by a motor driven blower whose output passes via an air delivery device to sealingly engage a patient's airway. A mask, tracheotomy tube, endotracheal tube, nasal pillows or other appropriate device may be used. An exhaust port is provided in a delivery tube proximate to the air delivery device. Some CPAP devices, termed bi-level CPAP, sense the breathing cycle of inspiration and expiration and provide different positive pressure levels during inhaling and exhaling. Some self-titrating CPAP devices are designed to determine appropriate pressure levels for the individual patient by selecting the least pressure that resolves the OSA. In such devices patterns of respiratory parameters are monitored to determine when OSA is present, and CPAP pressure is raised on the detection of appropriate patterns to provide increased airway pressure to, ideally, subvert the occurrence of obstructive episodes and the other forms of breathing disorder. Such devices are described in U.S. Pat. Nos. 5,148,802 and 5,245,995.
Typically a person suffering from OSA is diagnosed and treated in a sleep laboratory where the presence of the ailment is confirmed during a first night sleeping session and the appropriate treatment pressure is determined during a second night sleeping session. One problem that arises is that the appropriate pressure varies during the night as the person goes through different stages of sleep. Therefore there has been a long felt need for simplified apparatus for use in a patient's own home that could determine the presence of OSA and modify the pressure level to an optimum pressure. There have been several attempts, with varying success, to determine the presence of OSA from the analysis of the shape of airflow curves as a function of time.
The monitoring of upper airway pressure-flow relationships in obstructive sleep apnea has been described in Smith et al., 1988, J. Appl Physiol. 64: 789-795. FIG. 1 of that article shows polygraphic sleep recordings at varying levels of increasing nasal pressure. It was noted that inspiratory volumetric flow plateaued in certain breaths suggesting the presence of airflow limitation. Pressure-flow curves were constructed by plotting mid-inspiratory airflow against either mask pressure or endoesophageal pressure. The pressure-flow plots of nasal pressure against mean midinspiratory flow were then fit by least-squares linear regression to calculate resistance upstream to the collapsible-site.
The effect of positive nasal pressure on upper airway pressure-flow relationships has been described in Schwartz et al., 1989, J. Appl Physiol. 66: 1626-1634. FIG. 4 of the article shows that pressure-flow tracings plateau at a low pressure level. It was further shown when the pressure was increased, flow did not plateau.
U.S. Pat. No. 5,335,654 (Rapoport) shows the effect of CPAP on the airflow versus time curve for a patient suffering from OSA. FIGS. 1-5 in Rapoport show that as the pressure is reduced from 10 cm H2O to 2 cm H2O in steps of 2 cm H2O the curve of airflow versus time changes from an almost smooth sinusoidal pattern to one with a flattening of the inspiration portion of the curve with initial and terminal flow spikes. At 2 cm H2O the curve has developed a so-called M shape (i.e. with a ripple in the middle) and has also developed overshoots (i.e. peaks) at each end of the flattened central region. In an attempt to characterize the flow shapes that indicate obstruction, Rapoport lists several indices said to be indications of flow limitation and/or partial obstruction patterns including: (1) The derivative of the flow signal equals zero; (2) The second derivative between peaks of the flow signal is zero for a prolonged interval; (3) The ratio of early inspirational flow to mid-inspirational flow is less than or equal to 1. The patent further lists events said to be indications of obstructions: (1) Reduced slope of the line connecting the peak inspiratory flow to the peak expiratory flow; (2) Steep upward or downward stroke (dV/dt) of the flow signal; and (3) Ratio of inspiratory flow to expiratory flow over 0.5.
With regard to the control of CPAP treatment, various techniques are known for sensing and detecting abnormal breathing patterns indicative of obstruction. For example, U.S. Pat. No. 5,245,995 (Sullivan et al.) describes how snoring and abnormal breathing patterns can be detected by inspiration and expiration pressure measurements while sleeping, thereby leading to early indication of pre-obstructive episodes or other forms of breathing disorder. Particularly, patterns of respiratory parameters are monitored, and CPAP pressure is raised on the detection of pre-defined patterns to provide increased airway pressure to ideally prevent the occurrence of the obstructive episodes and the other forms of breathing disorder.
U.S. Pat. No. 5,645,053 (Remmers) describes calculating a flatness index, wherein flatness is defined to be the relative deviation of the observed airflow from the mean airflow. In Remmers, individual values of airflow are obtained between 40% and 80% of the inspiratory period. The mean value is calculated and subtracted from individual values of inspiratory flow. The individual differences are squared and divided by the total number of observations minus one. The square root of this result is used to determine a relative variation. The relative variation is divided by the mean inspiratory airflow to give a relative deviation or a coefficient of variation for that breath.
U.S. Pat. No. 5,704,345 (Berthon-Jones) disclosed a method for detecting partial obstruction of a patient's airway by calculating two obstruction index values that parameterize a flattening of an inspiratory portion of a patient's monitored respiratory airflow. Either obstruction index is then compared to a threshold value. The first shape factor involves a ratio of the mean of a mid-portion of the inspiratory airflow of the breathing cycle and the mean of the inspiratory airflow.
      shape    ⁢    1    =                    1        33            ⁢                                    ∑                          t              =              16                                48                ⁢                  fs          ⁡                      (            t            )                                M  where fs(t) is a sample of the patient inspiratory airflow and M is the mean of inspiratory airflow given by
  M  =            1      65        ⁢                  ∑                  t          =          1                65            ⁢                        fs          ⁡                      (            t            )                          .            
The second shape factor involves a ratio of the Root Mean Square deviation of a mid-portion of inspiratory airflow and the mean inspiratory airflow according to the formula
      shape    ⁢    2    =                                          1            33                    ⁢                                    ∑                              t                =                16                            48                        ⁢                                          (                                                      fs                    ⁡                                          (                      t                      )                                                        -                  M                                )                            2                                          M        .  
U.S. Pat. No. 6,814,073 (Wickham) discloses a method and apparatus for detecting some forms of obstruction based upon the inspiratory airflow. In this method, inspiratory airflow samples corresponding to mid-inspiration are identified. In one embodiment, weighting factors are applied based on whether the inspiratory airflow samples are less than or greater than a threshold level, such as the mean airflow. In another embodiment, different weighting factors are applied to samples based on their time positions in a breath. Samples taken prior to a certain event during inspiration, for example samples preceding the half way point of inspiration, are assigned lower weighting factors than samples succeeding the event. An obstruction index is then calculated using these samples with their corresponding weighting factors.
All these disclosed techniques fail to detect flow limitation in certain types of flow patterns, particularly the M-wave pattern. More generally, flow limitation in the inspiratory patterns that present with leading or lagging overshoot, is not appropriately detected. The detectability depends on two factors. 1) the extent to which the overshoot-traverses the mid-portion of the inspiratory airflow, and 2). the size of the overshoot. The solution presented by Wickham, works satisfactorily if the size of the overshoot is relatively small, does not span a large portion of mid-inspiration, and is not present in the latter half of the inspiratory airflow. However, flattening indices from the Wickham method are sometimes not as accurate as the indices from the method disclosed by Berthon-Jones. One aspect of the present invention is to simplify the algorithm mentioned in U.S. Pat. No. 5,704,345 (Berthon-Jones), the disclosure of which is incorporated by reference. Thus, an objective of the current invention is to present a method by which flow-limitation can be detected in the presence of the mentioned limitations, particularly the M wave pattern with overshoots.
The flow limitation detection/estimation technique described by the prior art is also expensive in terms of digital processing power, and the accuracy of flow detection required. Therefore its utility in low cost electronic/software platforms is limited. Therefore another objective of the present invention is to simplify the algorithm to render it more amenable to low-end electronic and software platforms.